1. Field of the Invention
The invention relates to a process for the location of cable faults, in particular for the location of non-burnout cable faults in the vicinity of branched cables, according to the pulse reflection method, in which a capacitor is charged and discharged across a spark gap by means of spark-over at a fault in the cable, and in which two measurements are superimposed in the vicinity of a digital storage unit to detect fault locations.
The invention also relates to an apparatus for the location of cable faults, in particular for the location of non-burnout cable faults in the vicinity of branched cables, according to the pulse reflection method, which apparatus has a rechargeable capacitor and a spark gap, connects the capacitor to the cable and has a digital storage unit for the recording and storage of two measurement curves.
2. Background Information
By means of such a process and such an apparatus, it is possible, in particular, to locate high and medium ohmic, non-burnout cable faults in an approximate manner. By discharging the capacitor across the spark gap, a reflection wave is generated in the vicinity of the cable fault, on the basis of which the distance to the fault can be determined.
In known arrangements, cable faults are located by means of high-voltage pulses from capacitor discharges and by displaying the reflection wave. The utility of this method is limited by the delay at the fault. Particularly, the propagation time, which is proportional to the distance to the fault location, can be extended to an unacceptable degree by the delay.
DE 41 00 305 A1 discloses a circuit configuration for the approximate location of high ohmic and intermittent cable faults using the pulse-echo method. An impulse wave generator together with a charging capacitor generates an impulse wave at a spark gap in air to strike a spark at the cable fault.
Additional methods of pulse location with high-voltage pulses are known, in which suitable pre-ionization of the fault renders the delay ineffective. Such methods are described in detail in the unexamined German patent specification DE 39 19 497.
Essentially, the object of unexamined patent specification DE 39 19 497 is a process in which the charge of a capacitor is connected to the faulty cable and an arc is produced at the fault location after the spark-over.
If the capacitor voltage is reduced to a certain value by the arc current, the remaining charge of the capacitor is discharged in pulses to the faulty cable by means of a short-circuit of the current limiting element, and the reflection wave thereby generated is input into a first channel of a digital storage unit to be recorded. Before or after this step, a reflection wave of a pulse voltage which is less than the trigger voltage, or ignition voltage of the fault, i.e., a reflection wave which does not trigger or ignite the fault, is input into a second channel of the digital storage unit to be recorded. The location of the fault is determined by superimposing the recordings of the two channels. This process is primarily suitable for high ohmic and intermittent faults.
On the other hand, medium ohmic faults, of the type which may occur if the fault is in water or in direct contact with damp ground, together with the current limiting element, form a voltage splitter which often reduces the voltage at the fault to such an extent that ignition no longer takes place. The voltage at the capacitor is therefore reduced by the current across the leakage resistance to the pulse ignition voltage without the fault having been ignited. The recordings in the digital storage unit thus contain essentially no information regarding the location of the fault.
Furthermore, known arrangements can tend to require a costly and complex apparatus for the rapid short-circuiting of the current limiting element. Sufficient auxiliary energy must generally be provided for the switching operation.
The capacitor of an impulse wave generator is used as the pulse capacitor. These capacitors are designed for high energy density, but do not necessarily have a low self-inductance. The individual sub-capacitors are also designed so that they can be switched for multiple voltages. The connecting lines to the selector switch increase the effective inductance. In the interest of a highly accurate location of the fault, the rate of change of the pulse should be as high as possible, and therefore the inductance of the circuit should be kept as low as possible.
The capacitor disclosed in DE-OS 39 19 497 is used to generate both the arc and the pulse. Consequently, only a portion of the original voltage charge is available as the pulse voltage. This can be a disadvantage under difficult ignition, or breakdown, conditions.